1. Field of the Invention
The present invention relates to a device and a method for forming an ornamental stitch in sewing machines, said device being of the type comprising a needle plate along which the workpiece is fed with a step-by-step movement according to a given sewing direction; at least two needles moving alternately backward and forward through the workpiece and the needle plate, following trajectories lying in vertical planes parallel to the sewing direction, said needles being each provided with a respective needle thread; a looper provided with a respective lower interlacing thread, cyclically movable along an elliptical elongate trajectory transversely of the sewing direction and extending below the needle plate to circumscribe the movement trajectories of the needles.
2. Prior Art
It is known that making seams with ornamental stitches, of the type identified in the United States Catalogue of Seams under Nos. 406 and 407 involves the use of at least two needles provided with respective needle threads, fastened in mutual side by side relation to a needle bar. Through said needle bar the needles are operated with a rectilinear reciprocating motion in a vertical direction, passing through a workpiece which is moved by a step-by-step movement in a given sewing direction over a needle plate provided with appropriate holes and/or cuts enabling passage of the needles.
Operating under the needle plate is a looper carrying a respective lower interlacing thread, mounted to the end of a supporting arm in turn fastened to a drive shaft disposed with an axis parallel to the sewing direction and substantially in alignment with an intermediate axis between the sewing needles.
By this drive shaft the looper is operated with a cyclic movement substantially describing an elloptical trajectory substantially extending in a horizontal plane below the needle plate, the major axis of which is oriented transversely of the sewing direction and circumscribes the movement trajectories of the needles.
To the ends of the present description, identified as "crossing" steps are those operating steps involved in the stitch formation in which the looper is slightly touched by the needles and vice-versa.
In more detail, for the stitch formation it is provided that the needles should carry out a down-stroke through the workpiece and the needle plate and, at the instant their up-stroke starts, should be "crossed" by the looper that enters corresponding looplike areas defined between the needles themselves and the respective needle threads. In this manner the needle threads, by the subsequent moving up of the needles, form respective loops that remain engaged in the looper. The lower interlacing thread carried by the looper is also inserted, together with said looper, into the loops formed by the needle threads and, being interlooped in the workpiece by effect of the previously formed sewing stitch, defines with the looper itself an insertion interspace having a substantially triangular conformation, the amplitude of which is correlated with the length of the sewing stitch, that is the movement pitch that is imposed to the workpiece when the needles, in the end step of their up-stroke and at the beginning of their new down-stroke, are extracted from the workpiece.
Concurrently with the workpiece displacement, the looper is slightly moved in a direction opposite to the sewing direction by an axial translation of the drive shaft, so that it is in side-by-side relationship with the needle trajectories at the front, that is on the opposite side from said looplike areas.
When the new down-stroke of the needles begins, said needles must slightly touch the looper while entering the triangular interspace defined by the looper itself with the respective lower interlacing thread, whereupon the looper is brought back to the starting position leaving the lower interlacing thread engaged around the needles and it is ready to enter again the above mentioned looplike areas as soon as the needles begin their new up-stroke.
From the foregoing it is apparent that at the present state of the art, the execution of the ornamental stitch has some limits. First of all, insertion of the needles in the triangular interspace formed by the looper and the lower interlacing thread is very critical, due to the reduced size of this interspace. As a result, in order to ensure a correct insertion of the needles, a slight interference must be necessarily produced between the looper and the needles during the looper crossing step, over an inclined surface arranged for the purpose.
These mechanical interferences however, inevitably give rise to important restrictions in the operating speed of the sewing machine, in that they cause overheating of the needles which will give rise to possible breaking of the same and/or burning of the threads. It is also to note that the needles, due to their slenderness are greatly affected by the effects of a combined bending and compressive stress when they operate at high speed, which will bring about, as a result, bendings and deformations the extent of which can be hardly established a priori. Adjustment of the interference between the needles and the looper is therefore even more difficult and critical, in spite of the aid of appropriate guide members such as the so-called "needle-pushers" and/or "needle-protecting elements".
It is also to note that, in order that a sufficient length of the needle threads may be drawn off during the needle descent, it is impossible to conveniently reduce the stroke performed by said needles to values under 30 mm. This condition represents another limitation to the operating speeds that can be achieved by the present sewing machines conveniently set up for making ornamental stitch seams. It is in fact apparent that a greater operating speed could be achieved, if said needles were able to perform reduced operating strokes.
Another problem presently found is given by the fact that the loops formed by the needles inevitably tend to be dragged along by the looper when the latter, during its return stroke, is about to be drawn out of the loops themselves. This situation causes the loops formed by the needle threads to be gathered together and mutually overlapped as a result of the looper movement, thereby giving rise to a seam of unacceptable quality or to breaking of the threads, above all when thread tensioning is not perfectly adjusted and/or when seams are made with the use of more than two needles and the distance between centers of these needles is reduced.